This invention relates to an adhesive composition and a method for temporarily bonding a removable flexible substrate to a rigid permanent substrate.
Construction of composite parts in which two or more parts are bonded with an adhesive to produce the final article, continues to be important, demanding development of higher performance composite materials and adhesives to bond these new composite materials.
Within this area, there is special need arising to remove selectively one of the adhered parts including total removal of the adhesive, such that the remaining part can be re-adhered to a fresh cover. Increasingly such type of cleanly strippable articles are important for specialist applications such as in aircraft interiors (where the strippable cover has additional features of high fire retardancy, low inflammability, low smoke emission etc.), or for construction of temporary signs where a rigid permanent support is covered with renewable signs output from various digital imaging devices (i.e. ink jet images for use on trucks and lorries, billboards etc.). Another area is in removal windscreens from automobile or other parts in the automobile applications where increasingly, bonded structures/articles are being incorporated.
Most important is that the bonded structures should have on the one hand significant peel resistance, yet the temporary (bonded) cover should still be removable with the simultaneous removal of the adhesive without damaging the remaining fixed part. This is especially important for aerospace application, where any damage to the fixed part can result in fracture points, deleterious to the aeroplane.
A mode of removing the adhered article involves the use of heating, be it from a hot air gun or other means.
Strippable adhesives are known, e.g. in the wall-paper area, where a combination of heat gun use and scrapping actions are used to removal haphazardly the wall paper. Clean strippability is not readily acheived.
Hot melt adhesives have been used, again not giving very clean separation between the adhered parts.
Two layer mechanisms have been suggested to achieve clean selective separation of adhered parts: U.S. Pat. No. 5,620,794, for example, discloses a two layer mechanism involving an adhesive and a heat active layer to separate a car windscreen from the car (metal) body.
U.S. Pat. No. 3,909,497 describes the heat decomposition of solid resins into flowing resins which could be the basis of Tg reduction and loss of adherence, however, clean separation in this mode is difficult.
Mixtures of acrylic adhesives with blowing agents having decomposition temperatures of 50-250xc2x0 C. are disclosed in JP-A 62-022874. On heating these compounds decompose forcing the adhering bonds to break. A clean selective removal of the adhesive cannot be achieved by this process.
It has now been found that an adhesive composition containing a linear epoxy resin, an amine hardener and an acetylene is suitable for bonding non-flexible parts (metal or non-metalic rigid supports) to flexible removable covers under mild curing conditions to achieve useful peel strengths, but on aggressive heating continues to bond preferentially to the removable part, such that substantially clean removal of the adhesive and the flexible removable part is effected.
Accordingly, the present invention relates to an adhesive composition comprising
(a) an epoxy resin having a substantially linear structure,
(b) an amine hardener, and
(c) a non-ionic surfactant.
Epoxy resins having a substantially linear structure can be prepared by an advancement reaction, i.e. by polymerizing a difunctional epoxy resin having two epoxy groups per molecule and a dihydric phenol being used in quantities which provide a ratio of phenolic hydroxyl groups to epoxy groups of from 1:0.8 to 1:1.2. Suitable starting materials and reaction conditions for the advancement reaction are well-known in the art.
Examples for suitable difunctional epoxy resins are diglycidyl ethers or diglycidyl esters like bisphenol-A-diglycidylether, bisphenol-F-diglycidylether, hydrogenated bisphenol-A-diglycidylether, hydrogenated bisphenol-F-diglycidylether and diglycidyl esters of phthalic, isophthalic or terephthalic acid.
Difunctional cycloaliphatic epoxy resins like 3,4epoxycyclohexylmethyl-3xe2x80x2,4xe2x80x2-epoxycyclohexanecarboxylate can also be applied.
Bisphenol-A-diglycidyl ether and bisphenol-F-diglycidyl ether are particularly preferred.
Examples for dihydric phenols which can be used for the advancement reaction of difunctional epoxy resins are 4,4xe2x80x2-dihydroxybiphenyl, bisphenol A, bisphenol F, bisphenol S and 4,4xe2x80x2-dihydroxydiphenyl ketone.
Bisphenol A is the preferred dihydric phenol.
Amine hardeners which can be used as component (b) in the compositions according to the invention are preferably aliphatic and cycloaliphatic amines.
The aliphatic amine may be alkylene diamines such as ethylene or propylene diamine, triethylene diamine, piperazine-N-ethylamine, polyoxyalkylene diamines, such as polyoxyethylene diamine or polyoxypropylene diamine and the like. Cycloaliphatic amines may be used as cyclohexane diamine, isophorone diamine and the like.
Tertiary amines and imidazoles are preferably employed as curing catalysts. Suitable examples are benzyldimethylamine, 2,4,6-tris(dimethylaminomethyl)phenol, 1-methylimidazole, 2-methylimidazole and 2,4-diethylimidazole.
Basically, every non-ionic surfactant can be applied as component (c). Suitable surfactants are, for instance, fluorosurfactants like derivatives of non-ionic perfluorinated polyalkylenes and polyoxyalkylenes, alkoxylated, in particular ethoxylated, perfluorinated fattic acid derivatives, polyglycidylethers of castor oil, melamine derivatives, like hexamethoxymethylmelamine, or compounds of formula I,
R1xe2x80x94Cxe2x89xa1Cxe2x80x94R2xe2x80x83xe2x80x83(I) 
wherein R1 and R2 independently from one another denote hydroxy or an unsubstituted or hydroxy or halogen-substituted organic radical selected from the group consisting of C1-C20alkyl, C1-C20alkoxy, C5-C20aryl, C5-C20aryloxy, C6-C20aralkyl, C5-C12cycloalkyl, C5-C12cycloalkoxy and C2-C20-alkoxyalkyl.
The compositions according to the invention may further contain stabilizers known in the art. The stabilizers may be for protection during the life of the finished product against, for example, oxygen, ozone, and UV radiation.
Compounds of formula I wherein R1 and R2 denote Ethoxy are especially preferred.
Environmental standards nowadays require to minimize the use of organic solvents in adhesive compositions and therefore there is a special need for water-based adhesives. The compositions according to the invention are highly water dispersible and sprayable and accordingly is particularly suitable for adhering low energy (composite) surfaces to rigid metal or non-metal composite surfaces or parts.
The invention therefore further relates to an aqueous dispersion containing an adhesive composition containing an epoxy resin having a substantially linear structure, an amine hardener and a non-ionic surfactant.
The adhesive composition is used as a two-component composition, i.e. the epoxy resin (Part A) and the amine hardener (Part B) are stored separately and mixed a short time prior to application. The non-ionic surfactant can be added either to the epoxy or to the hardener component. Water can be added as well to either part of the composition to adjust the viscosity.
The adhesive compositions have almost no or only a very mild odor. The viscosities of Part A and Part B are low and similar thus allowing easy mixing. The formulation can be applied to part using any type of spray equipment or roller coat; no slumping, sagging or mottling of adhesive appears during application. The water in the formulation evaporates quickly; the adhesive is milky white when sprayed on the substrate and turns opaque when water has flashed off thus visually telling the operator that the sides can be bonded together.
The adhesive can be used in high humidity environments as well as in low and high temperature environments (10-40xc2x0 C.), has a long pot life and can be used on any low surface tension substrates. The used equipment can be cleaned with water. The cured adhesive distinguishes by high strength and is easily strippable by application of heat (for example by a heat gun).
The cured adhesive panels can withstand high temperature and high humidity environments (7 days at 40xc2x0 C. with 90% relative humidity).
Furthermore, the composition according to the invention can be used as binder for grit to paper for sand paper applications.
The invention provides water based adhesives which can achieve the required good peel strength in active use to bond the two parts together in a simple manner applicable in the field (i.e. trade shop or workplace) under environmentally friendly conditions, yet be totally removed with the removable xe2x80x98temporaryxe2x80x99 cover, on heating in a single step in a clean manner, leaving no residue to remove laboriously, as done in the systems in the art.
Surprisingly, substantially all the adhesive is removed on heat peeling the removable cover, leaving a clean flat surface on the permanent part, which is then ready for the next cover to be applied. This is an unexpected result, as usually the heated side should be where the adhesive failure should be, as is the case with hot melt adhesives.
A further object of the invention is a method for temporarily bonding a removable flexible substrate to a rigid permanent substrate which comprises,
(A) applying a composition according to claim 1 to the surface of the removable substrate or to the surface of the permanent substrate or to both surfaces,
(B) bringing both substrates together and curing the adhesive composition, and
(C) after the useful lifetime of the removable part separating the removable substrate together with the adhesive by exposing the array to temperatures above 160xc2x0 C.
The permanent substrate is preferably aluminium, steel, a graphite composite, a glass composite or a phenolic composite.
The removable substrate may be any flexible (transparent, opaque, imaged, or decorated) polymer film with or without specialist properties like high fire retardancy, low smoke emission and good thermal stability. It should be highly durable, washable, and steam-cleanable. It should also be impervious to common solvents, acids, greases and other chemicals.
The claimed method is particularly useful for adhering and removing parts which essentially consist of a poly(fluoroalkene), preferably polyvinylidene fluoride.
Polyvinyl fluoride films, like for example TEDLAR(copyright) (DuPont) are particularly preferred.